The present invention relates to a catalyst for purifying exhaust gases from an internal combustion engine. In particular, it relates to a lean NOx catalyst.
It is well known in the art to use catalyst compositions, including those commonly referred to as three-way conversion (“TWC”) catalysts to treat the exhaust gases of internal combustion engines. Such catalysts, containing precious metals like platinum, palladium, and rhodium, have been found both to successfully promote the oxidation of unburned hydrocarbons (HC) and carbon monoxide (CO) and to promote the reduction of nitrogen oxides (NOx) in exhaust gas, provided that the engine is operated around balanced stoichiometry for combustion (“combustion stoichiometry”; i.e., between about 14.7 and 14.4 air/fuel (A/F) ratio).
However, fuel economy and global carbon dioxide (CO2) emissions have made it desirable to operate engines under lean-burn conditions, where the air-to-fuel ratio is somewhat greater than combustion stoichiometry to realize a benefit in fuel economy. Diesel and lean-burn gasoline engines generally operate under highly oxidizing conditions (i.e., using much more air than is necessary to burn the fuel), typically at air/fuel ratios greater than 14.7 and generally between 19 and 35. Under these highly lean conditions, typical three-way catalysts exhibit little activity toward NOx reduction, as their reduction activity is suppressed by the presence of excess oxygen.
The control of NOx emissions from vehicles is a worldwide environmental problem. Lean-burn, high air-to-fuel ratio, and diesel engines are certain to become more important in meeting the mandated fuel economy requirements of next-generation vehicles. Development of an effective and durable catalyst for controlling NOx emissions under net oxidizing conditions accordingly is critical.
Recently, copper-ion exchanged zeolite catalysts have been shown to be active for selective reduction of NOx by hydrocarbons in the presence of excess oxygen. Platinum-ion exchanged zeolite catalyst is also known to be active for NOx reduction by hydrocarbons under lean conditions. However, this catalytic activity is significant only in a narrow temperature range around the lightoff temperature of hydrocarbon oxidation. All the known lean-NOx catalysts reported in the literature tend to lose their catalytic activity for NOx reduction when the catalyst temperature reaches well above the lightoff temperature of hydrocarbon oxidation. This narrow temperature window of the lean-NOx catalysts is considered to be one of the major technical obstacles, because it makes practical application of these catalysts difficult for lean-burn gasoline or diesel engines.). As an example, the Cu-zeolite catalysts deactivate irreversibly if a certain temperature is exceeded. Catalyst deactivation is accelerated by the presence of water vapor in the stream and water vapor suppresses the NO reduction activity even at lower temperatures. Also, sulfate formation at active catalyst sites and on catalyst support materials causes deactivation. Practical lean-NOx catalysts must overcome all three problems simultaneously before they can be considered for commercial use. In the case of sulfur poisoning, some gasoline can contain up to 1200 ppm of organo-sulfur compounds. Lean-NOx catalysts promote the conversion of such compounds to SO2 and SO3 during combustion. Such SO2 will adsorb onto the precious metal sites at temperatures below 300° C. and thereby inhibits the catalytic conversions of CO, CxHy (hydrocarbons) and NOx. At higher temperatures with an Al2O3 catalyst carrier, SO2 is converted to SO3 to form a large-volume, low-density material, Al2(SO4)3, that alters the catalyst surface area and leads to deactivation. In the prior art, the primary solution to this problem has been to use fuels with low sulfur contents.
Another alternative is to use catalysts that selectively reduce NOx in the presence of a co-reductant, e.g., selective catalytic reduction (SCR) using ammonia or urea as a co-reductant. Selective catalytic reduction is based on the reaction of NO with hydrocarbon species activated on the catalyst surface and the subsequent reduction of NOx to N2. More than fifty such SCR catalysts are conventionally known to exist. These include a wide assortment of catalysts, some containing base metals or precious metals that provide high activity. Unfortunately, just solving the problem of catalyst activity in an oxygen-rich environment is not enough for practical applications. Like most heterogeneous catalytic processes, the SCR process is susceptible to chemical and/or thermal deactivation. Many lean-NOx catalysts are too susceptible to high temperatures, water vapor and sulfur poisoning (from SOx).
Yet another viable alternative involves using co-existing hydrocarbons in the exhaust of mobile lean-bum gasoline engines as a co-reductant and is a more practical, cost-effective, and environmentally sound approach. The search for effective and durable non-selective catalytic reduction “NSCR” catalysts that work with hydrocarbon co-reductant in oxygen-rich environments is a high-priority issue in emissions control and the subject of intense investigations by automobile and catalyst companies, and universities, throughout the world.
A leading catalytic technology for removal of NOx from lean-burn engine exhausts involves NOx storage reduction catalysis, commonly called the “lean-NOx trap”. The lean-NOx trap technology can involve the catalytic oxidation of NO to NO2 by catalytic metal components effective for such oxidation, such as precious metals. However, in the lean NOx trap, the formation of NO2 is followed by the formation of a nitrate when the NO2 is adsorbed onto the catalyst surface. The NO2 is thus “trapped”, i.e., stored, on the catalyst surface in the nitrate form and subsequently decomposed by periodically operating the system under stoichiometrically fuel-rich combustion conditions that effect a reduction of the released NOx (nitrate) to N2.
The lean-NOx-trap technology has been limited to use for low sulfur fuels because catalysts that are active for converting NO to NO2 are also active in converting SO2 to SO3. Lean NOx trap catalysts have shown serious deactivation in the presence of SOx because, under oxygen-rich conditions, SOx adsorbs more strongly on NO2 adsorption sites than NO2, and the adsorbed SOx does not desorb altogether even under fuel-rich conditions. Such presence of SO3 leads to the formation of sulfuric acid and sulfates that increase the particulates in the exhaust and poison the active sites on the catalyst. Attempts with limited success to solve such a problem have encompassed the use of selective SOx adsorbents upstream of lean NOx trap adsorbents. Furthermore, catalytic oxidation of NO to NO2 is limited in its temperature range. Oxidation of NO to NO2 by a conventional Pt-based catalyst maximizes at about 250° C. and loses its efficiency below about 100 degrees and above about 400 degrees. Thus, the search continues in the development of systems that improve lean NOx trap technology with respect to temperature and sulfur considerations.
Another NOx removal technique comprises a non-thermal plasma gas treatment of NO to produce, NO2 which is then combined with catalytic storage reduction treatment, e.g., a lean NOx, trap, to enhance NOx reduction in oxygen-rich vehicle engine exhausts. In the lean NOx trap, the NO2 from the plasma treatment is adsorbed on a nitrate-forming material, such as an alkali material, and stored as a nitrate. An engine controller periodically runs a brief fuel-rich condition to provide hydrocarbons for a reaction that decomposes the stored nitrate into benign products such as N2. By using a plasma, the lean NOx trap catalyst can be implemented with known NOx adsorbers, and the catalyst may contain less or essentially no precious metals, such as Pt, Pd and Rh, for reduction of the nitrate to N2. Accordingly, an advantage is that a method for NOx emission reduction is provided that is inexpensive and reliable. The plasma-assisted lean NOx trap can allow the life of precious metal lean NOx trap catalysts to be extended for relatively inexpensive compliance to NOx emission reduction laws. Furthermore, not only does the plasma-assisted lean NOx trap process improve the activity, durability, and temperature window of lean NOx trap catalysts, but it allows the combustion of fuels containing relatively high sulfur contents with a concomitant reduction of NOx, particularly in an oxygen-rich vehicular environment.
What is needed in the art is an exhaust gas catalyst system having improved durability, as well as effective NOx management, over extended operating time. The present invention overcomes many of the shortcomings of the prior art.